Recently, a demand for ultra fine particles having a particle diameter of submicrons is increasing in many industrial fields (for instance, high technology fields as well as fine ceramics fields). As one means for meeting this demand, the use of a media agitation mill has come to attention in view of its advantage of cost savings. The media agitation mill uses beads (sometimes called balls, media or ball pebbles) as grinding media. As a material thereof, a metal, glass or ceramic has been mainly used. However, beads made of a metal or glass formed during the grinding step or abrasive particles or impaired peeled pieces thereof generated by abrasion or cracking contaminate a final product to cause pollution, resulting in deterioration of quality and irregular quality. Thus, since they directly and adversely affect the final product, it has come to attention to use ceramic beads, especially zirconia beads in which an yttria stabilizer is contained, which is less influenced by the above factors, and the use thereof is increasing.
Conventionally, in cases where beads are used as a media for grinding particles, it is said that it is better to use beads having a high density, a small average particle diameter, a narrow distribution breadth and a nearly spherical shape. Accordingly, a demand on the market is increasing for beads made of, e.g., zirconia or other ceramic materials, having a high density (when it is the same ceramics, the nearer the theoretical density is better), a small average particle diameter, a narrow distribution breadth and a nearly spherical shape. In particular, since zirconia beads have higher density than those of other ceramic materials and are abundant in abrasion resistance, it is said that a demand for beads made of zirconia having a smaller shape, narrower particle diameter distribution breadth and more nearly spherical shape will become stronger from now on.
Beads having a small particle diameter (e.g., 200 .mu.m or 300 .mu.m) made of a metal or glass as a material have been already on the market, and they have a relatively high sphericity. Zirconia beads having an average particle diameter of 400 .mu.m are obtainable on the market and put in practical use as grinding media. Also, it is possible to obtain zirconia beads having an average particle diameter of 300 .mu.m, however, in cases of those having an average particle diameter of 300 .mu.m, the density thereof is 6.0 g/cm.sup.3 or smaller, the particle diameter distribution thereof is broad (25 to 30 .mu.m in the standard deviation), and the sphericity thereof is 1.1 or higher, which are not sufficient levels. It is considered that these disadvantages are attributed to conventional granulating methods such as a rolling method, a fluidized bed method, or an agitation method. Accordingly, the present inventors have found that 1) there is a technical possibility to produce zirconia beads having an average particle diameter of 400 .mu.m or smaller, high density, narrow particle distribution and good sphericity and 2) it is preferred to use the beads as grinding media in agitation mills.